1. Field of the Invention
The present invention relates to creping a cellulosic web from a rotatable cylinder to form paper, such as toilet tissue, facial tissue, and paper toweling, for example. More particularly, the present invention relates to a creping blade and system for creping a cellulosic web from a rotatable cylinder. The present invention also relates to a method of manufacturing paper and paper having substantially constant caliper and strength.
2. Description of Related Art
Paper is generally manufactured by a process that includes dispersing cellulosic fibers (e.g., wood fibers) in a liquid (e.g., solution including water) to form a mixture having the cellulosic fibers suspended in the liquid. A substantial portion of the liquid is then removed from the mixture. As the liquid is removed, the cellulosic fibers begin to link to one another, thereby forming a cellulosic web. The linking of the cellulosic fibers results from mechanical interlocking of the fibers and from hydrogen bonding between the fibers. The hydrogen bonding between the fibers is the predominant linking mechanism.
After removing at least a portion of the liquid from the mixture, the cellulosic web is positioned on a rotatable cylinder, such as a heated Yankee dryer, to remove more of the liquid from the mixture. Depending on the amount of liquid still present, the cellulosic web either is self-adhered to the rotatable cylinder or is positioned on the rotatable cylinder with an adhesive agent configured to allow removal of the web from the cylinder without destroying the web. After the web has been rotated on the cylinder to remove additional moisture, the web is removed from the rotatable cylinder. Thereafter, the web is either wound onto a reel or may be further dried and processed into paper and/or paper products.
The structural integrity and strength of the cellulosic web results from the mechanical and hydrogen bonding between the individual cellulosic fibers. Strength and softness of the paper, however, are inversely proportional to one another. That is, as the strength of the paper increases, the softness of the paper decreases. For paper that is used as bathroom tissue (e.g., toilet tissue or facial tissue), both strength and softness are very important. In particular, consumer preferences demand soft bathroom tissue.
Paper produced by conventional processes, such as the process described above, is generally perceived by consumers as not being soft enough for use as bathroom tissue. One common method of increasing the softness of paper used as bathroom tissue is to crepe the paper. Creping is a procedure that includes scraping the cellulosic web from the rotatable cylinder with a creping blade. Creping the cellulosic web advantageously breaks some of the inter-fiber bonds of the cellulosic web, thereby increasing the softness and decreasing the strength of the paper.
Conventional creping blades generally include an elongated blade having a planar, beveled surface that defines a scraping edge. The blade is generally substantially the same length as the rotatable cylinder. The scraping edge is positioned against the rotatable cylinder to scrape the cellulosic web from the cylinder to break some of the inter-fiber bonds, and thereby increase the softness. Creping also increases the caliper of the cellulosic web. Caliper, as used herein, is a term of art that refers to the thickness or bulk of paper. Convention creping blades, however, suffer from the draw-back that the caliper of paper produced by them is still not large enough.
A modified creping blade that produces bathroom tissue having a larger caliper than conventional creping blades, while maintaining a desirable level of strength and softness of the paper, is disclosed in U.S. Pat. No. 5,656,134 (hereafter xe2x80x9cthe ""134 patentxe2x80x9d), the entire disclosure of which is incorporated herein by reference. The ""134 patent discloses a creping blade (hereafter xe2x80x9cthe ""134 bladexe2x80x9d) that includes a beveled surface beveled with respect to faces of the blade and serrulations formed in the blade adjacent to the bevel surface. The serrulations are preferably configured so that a bottom of each serrulation is perpendicular to faces of the blade. The serrulations advantageously provide paper having a desired combination of strength, softness, and caliper or thickness, for use as bathroom tissue. See the ""134 patent, column 3, line 26 to column 4, line 6.
To crepe a cellulosic web, the ""134 blade is positioned on a rotatable cylinder (e.g., Yankee dryer) so that a scraping edge or surface will scrape the cellulosic web from the cylinder when the cylinder rotates with the cellulosic web thereon. The blade is positioned with respect to the cylinder at an angle called a wear or creping angle. The wear or creping angle is defined as an angle having a vertex at the point of contact between the blade and the cylinder and rays defined by a portion of a face of the blade and a portion of a line tangent to the point of contact.
The caliper of the paper produced with the ""134 blade is determined in part by an effective depth of the serrulations. The effective depth is defined as the depth of the serrulations measured along the wear angle (i.e., along the direction of a line tangent to the cylinder at the blade contact point). As the blade disclosed in the ""134 patent wears, the effective depth of the serrulations changes. When the depth of the serrulations changes, the caliper and strength of the paper produced using the serrulated blade also changes. At a point where the caliper and strength of the paper produced by a blade configured like the ""134 blade is no longer within acceptable manufacturing tolerances because of the changing effective serrulation depth, the creping blade must be replaced.
The amount of production time during which a creping blade will produce saleable paper (i.e., paper having a caliper and strength within manufacturing tolerances) before being replaced is referred to as the useful life of the blade. The actual useful life of a blade depends upon a number of factors, such as the material in the cellulosic web. For example, recycled material, such as material including ash, tends to wear creping blades faster than other types of materials.
It is advantageous to have a creping blade with a relatively long useful life because creping blade replacement is extremely costly. In particular, the entire production line must be shut down every time the creping blade is replaced and during this shut down time no saleable paper can be produced. In addition, creping blades are relatively expensive to produce.
In light of the foregoing, there is a need in the art for an improved creping blade, an improved system for creping a cellulosic web, and an improved method for creping a cellulosic web.
Accordingly, the present invention is directed to a creping blade, a system for creping a cellulosic web, and a method of manufacturing paper that obviate one or more of the shortcomings of the related art. To achieve these and other advantages, and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention includes a creping blade for creping a cellulosic web from a rotatable cylinder in a creping process. The creping blade includes first and second side faces. The first side face is at least substantially opposite to the second side face. The blade also includes an upper surface that is not orthogonal to at least one of the first and second side faces. Also included are a plurality of notches. Each of the notches has a bottom portion and an open end. The bottom portion is at least substantially parallel to the upper surface and the open end is defined by at least a portion of the upper surface. The notches are configured to increase the caliper of the cellulosic web when the creping blade crepes the cellulosic web from an outer surface of the rotatable cylinder.
In an aspect, the upper surface is planar.
In another aspect, an effective notch depth of each notch, which is defined by the distance between the bottom portion and the open end in a direction along a wear angle of the creping blade, remains substantially constant when contact between the creping blade and the rotatable cylinder wears the creping blade. Preferably, the caliper and strength of the cellulosic web creped by the creping blade are substantially unaffected by wear of the creping blade.
In yet another aspect, the notches are configured so that the bottom portion of each of the notches is at least substantially in a plane that is at least substantially parallel to the upper surface.
In still another aspect, the bottom portion of at least one of the notches is in a first plane, the bottom portion of at least another of the notches is in a second plane, and the upper surface is in a third plane. The first, second, and third planes are at least substantially parallel to one another and a distance between the first and third planes is different from a distance between the second and third planes.
In a further aspect, the creping blade includes a plurality of protrusions adjacent to the notches and extending from at least one of the first and second side faces. At least a portion of the plurality of the protrusions defines at least a part of the creping blade that contacts the outer surface of the rotatable cylinder. The protrusions are preferably formed from portions of the creping blade displaced when the plurality of notches are formed. More preferably, outer faces of the protrusions are dressed to an angle with respect to at least one of said first and second side faces approximately equal to a wear angle of the creping blade when the creping blade is positioned on the outer surface of the rotatable cylinder.
In another aspect, the invention includes a system for creping a cellulosic web. The system includes a rotatable cylinder and a creping blade similar to one of the creping blades described above. The creping blade is positioned with respect to the cylinder so that the creping blade is capable of creping cellulosic web from an outer surface of the cylinder when the web is on the outer surface and the cylinder is rotated.
In a further aspect, the system includes a pivot member coupled to the creping blade. The pivot member is configured to maintain the creping blade in contact with the outer surface of the cylinder when the creping blade becomes worn.
In yet another aspect, the invention includes an improvement to a method of manufacturing paper. The improvement includes creping a cellulosic web from an outer surface of a rotatable cylinder with a creping blade similar to one of the creping blades described above. The caliper and strength of the creped web is substantially constant when contact between the creping blade and the outer surface of the rotatable cylinder wears the creping blade.
In still another aspect, the invention includes paper having substantially constant caliper and strength produced by the improved method of manufacturing described above.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.